The background art of the present invention will be described with an illustration of an excavator shown in FIG. 6.
The excavator includes a crawler-type lower travel body 1, an upper slewing body 2 mounted thereon so as to be able to be slewed about the X-axis perpendicular to the ground, and an excavation attachment 3 attached to the upper slewing body 2. The excavation attachment 3 includes a boom 4 capable of being raised and lowered, an arm 5 attached to a distal end of the boom 4, a bucket 6 attached to a distal end of the arm 5, and respective hydraulic cylinders for actuating the boom 4, the arm 5, and the bucket 6, namely, a boom cylinder 7, an arm cylinder 8, and a bucket cylinder 9.
As a slew driving system for driving to slew the upper slewing body 2 of such an excavator, there is known one described in Patent Document 1. The shown slew driving system includes: a hydraulic motor for slewing, as a drive source; an electric motor connected to an output shaft of the hydraulic motor, a control valve, a communication valve, which is a solenoid switching valve provided between motor both-side lines provided on both sides of the hydraulic motor respectively and the control valve, the communication valve being capable of bringing the motor both-side lines into direct communication with each other; and an electric storage device. In the slew driving system, the communication valve is switched, upon slew braking, i.e., upon deceleration, so as to return discharged oil from the hydraulic motor to the inlet side of the hydraulic motor, and the electric motor is controlled to make a generator action for generating regenerative power generation and a regenerative brake action. The regenerative power thus generated is stored in the electric storage device.
In this system, the communication valve reduces the back pressure which acts on the motor outlet side when the slew is braked, by the direct communication between the motor both-side lines, to reduce the load of the hydraulic motor due to the involvement rotation thereof, thereby enhancing the efficiency in the recovery of the inertial motion energy, i.e., regenerative efficiency. However, in the case of abnormal switching of failing to operate the communication valve in accordance with commands due to disconnection in a control system for switching control of the communication valve or sticking of a spool or the like, various slewing troubles can be generated. For example, an occurrence where a communication valve is disabled from return from an open position to a close position prevents drive force for the hydraulic motor from being exerted and also prevents the holding force by hydraulic pressure from being exerted; this generates a risk of failing to slewing and further downward slewing due to gravity on a slope in spite that upward slewing should be performed. On contrary, an occurrence where the communication valve is disabled from being switched from the close position to the open position prevents the motor braking torque from being exerted in spite of counter operation applied to an operation member, such as a lever, for slew braking during slewing; this causes a risk of leaving a slewing body to continue inertial slewing.
Although Patent Document 1 discloses a brake valve formed of a pair of relief valves and the like, which is provided between the motor both-side lines, the brake valve is not activated during slew braking and only performs a function of keeping stop of slewing immediately after the stop thereof.